Selective Anti-Stokes Excitation of a Single Defect Center in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) with defect centers is attracting attention as a promising candidate for single-photon emitters because its emission spectrum shows sharp emission peaks and it exhibits ultrabright single-photon emission at room temperature. However, when hBN containing multiple defect centers with various transition frequencies is excited using a pump laser with a wavelength shorter than the emission wavelength of the target defect center, problems such as a broad emission spectrum and nonsingle photon emission often arise. In this paper, we present a method for selectively exciting a single defect center in hBN nanoflakes by anti-Stokes excitation, where the wavelength of the excitation laser is longer than the emission wavelength of the target defect center. By exciting an hBN nanoflake with multiple defect centers using a 637 nm laser, as opposed to the normal excitation using a 532 nm laser, we observed a sharp emission peak and an antibunching dip (g2(0) = 0.08 ± 0.05) in the second-order correlation function, indicating strong evidence of a single defect center. In addition, we explored the temperature dependence of the emission spectrum from the defect centers in a hBN nanoflake to clarify the mechanism of the anti-Stokes excitation. As a result, we confirmed that single-phonon-mediated anti-Stokes excitation plays a substantial role in suppressing shorter-wavelength emissions and improving g2(0). Because this method uses a long-wavelength laser for excitation, it is speculated to substantially reduce the background photons generated not only inside the hBN nanoflakes but also in the optical fiber guiding the pump light, which is often a severe problem.